1. Technical Field of the Invention
The present invention relates generally to brushless automotive alternators for use in, for example, trucks, construction machines, and agricultural machines.
More particularly, the invention relates to a brushless automotive alternator which has an improved structure for minimizing the temperature of auxiliary rectifying elements included therein.
2. Description of the Related Art
With recent development of automotive engines for coping with emission control regulations, automotive alternators have come to operate under higher ambient temperatures in the engine compartments.
Moreover, to meet recent requirements for employing more security devices, providing additional power sources, and improving comfort, electric loads on automotive alternators have considerably increased.
There are two different automotive alternator types. One is brushed type for passenger cars, and the other is brushless type for trucks, construction machines, and agricultural machines.
In general, trucks are required to be able to travel long distances, and construction machines and agricultural machines are required to be able to operate under severe conditions (e.g., dusty conditions).
Accordingly, brushless automotive alternators are now required to be able to withstand higher ambient temperatures, output more power, and have a long service life.
To increase power output of automotive alternators, several techniques have been developed, for example, increasing the outer diameter of the stator, increasing the lamination thickness of the stator core, and lowering the resistance of the stator winding.
However, with increase in the power output of an automotive alternator, heat generated in the rectifier of the alternator increases accordingly, thus causing the temperature of the rectifier to significantly increase.
Japanese Patent First Publication No. S60-109748 discloses an automotive alternator, which includes a rectifier having main diodes for providing DC power output of the alternator and auxiliary diodes for providing DC field current to the field winding. The auxiliary diodes are arranged on a main (top or bottom) surface of a heat sink of the rectifier.
However, with such an arrangement, when the temperature of the rectifier increases with increase in the power output of the alternator, the temperature of the auxiliary diodes increases accordingly. This is because, though the auxiliary diodes generate only a small amount of heat by themselves, they receive a large amount of heat transferred from the heat sink which carries the main diodes thereon.
On the other hand, for brushless automotive alternators, especially for those for North American market which have a normalized dimension of stays for installation, the alternator outer diameter is generally limited, and thus the distances of a bearing supporting the rotor with the stator and with the rectifier are accordingly limited. Consequently, the temperature of the bearing also increases with increase in the power output of the alternator.
Accordingly, due to the auxiliary diodes and bearing whose temperatures increase with increase in the power output, it is difficult to secure a long service life of a brushless automotive alternator.
To suppress temperature rises of the auxiliary diodes and bearing, there has been disclosed a method of increasing the airflow rate and/or speed of a cooling fan provided in the alternator. However, due to the limitation on the alternator outer diameter as described above, there is accordingly a limitation on increasing the outer diameter of the cooling fan. Further, due to constraints on the strength of blades of the cooling fan, there is also a limitation on increasing the number of blades and/or axial area of the cooling fan.
Accordingly, there is a limitation on suppressing temperature rises of the auxiliary diodes and bearing by using the above method.